Scrambling multimedia contents makes it possible to subject the descrambling of these multimedia contents to the acquisition, through payment, of an access right whose validity is checked each time the multimedia contents are accessed.
In this context, it is also necessary to protect the multimedia contents transmitted in scrambled form and then recorded. In practice, if the possibility of recording the multimedia content is left without read protection, then said multimedia content can be reused and reread indefinitely by the holder of the rights, and freely made available, and usable in readable form by other users who have not acquired the access rights required to view this multimedia content. Now, these days, it is very easy to broadcast a recorded multimedia content to a large number of people, notably via sharing networks. For example, a sharing network is a station-to-station network, better known as “peer-to-peer” network. In another alternative, the sharing is done by using hosted servers.
It has therefore already been proposed to record multimedia contents in scrambled form. Thus, the applicant knows a method for protecting a recorded multimedia content in which:    a) a network head broadcasts on a channel a scrambled multimedia content and ECM (Entitlement Control Message) messages containing cryptograms CWKa of control words CW enabling each to descramble a respective cryptoperiod of the scrambled multimedia content,    b) any one of the recorders receives the scrambled multimedia content and the ECM messages and decrypts the cryptogram CWKa contained in the ECM message received with a subscription key Ka and read-protects the scrambled multimedia content using a key KHe by encrypting the decrypted control words with the local key KHe to generate cryptograms CWKHe,    c) the recorder records the cryptograms CWKHe and the scrambled multimedia content with the control words CW.
In most cases, the recorder is implemented in a terminal inside which a reader is also implemented which enables the recorded multimedia content to be read or played in clear.
The expression “in clear” denotes the fact that the multimedia content read is directly perceptible and comprehensible to a human being. In other words, the in clear multimedia content is the result of a correct descrambling of the scrambled multimedia content.
In these known methods, the local key KHe is generated locally by the terminal and kept secret in a security processor. Thus, only the reader of this terminal can play in clear the recorded multimedia content protected by means of this key.
State of the art is also known from:                US2005/262529A1,        US2008/253564A1,        EP1575291A2,        WO2007/146763A2,        US20041194125A1.        